for me nothing, and i mean nothing, slaps on the mass better than barbell squats. i can leg press, hack squat, and smith squat until the cows come home and my legs will simply just not respond like they do when i just do barbell squats. the only downside is that my knees tend to get a bit sore after a while, but i honestly think that is due to using too much volume in the past.
i have had to switch to the leg press because of a minor knee injury the past two months and my legs have lost a LOT of size.
if you think about it, when i do barbell squats, i are not only working my quads, hams, and glutes, but also all the assistor muscles that are required to maintain balance throughout the movement (adductors, hip flexors, etc.). building these muscles up adds up to greater leg size than what i would get out of the leg press for example, where all i have to do is push.
squats just do something to your body as a whole that cannot be substituted for, at least not in my case. they cause a complete shift in my physique.